1. Field of the Invention
The present invention relates to an expandable resin composition and, more particularly, to a thermoplastic olefin resin composition containing solid fillers suitable for use in puff molding of shaped parts.
2. Description of the Prior Art
Puff molding is a process that results in a molded plastic foam shape significantly larger than the confining mold in which it was formed. A thermoplastic resin is blended with certain blowing and cross-linking agents. As the temperature is increased the blowing agent decomposes to release gas while simultaneously the cross-linking agent forms bonds between the molecules of the thermoplastic resin. When these changes occur within a heated, confining mold, the result is a compacted, cross-linked plastic matrix having memory of the shape of the confining mold, and filled with microscopic bubbles of highly compressed gas. If the hot mold is suddenly opened, the shape instantly expands to many times is original volume, but because of the cross-linking that has occurred the expanded shape retains the details and general configuration of the mold.
Puff molding has usually been utilized to form industrial parts such as shoe soles, insulation, trim, edging and other articles where tolerances were wide, the surface appearance and feel was not important to its use and the ability to hold intricate contours was not of concern. Prior processes required long residence times from loading to removal of the part from the mold and usually required cooling of the mold surfaces to affect release of the part from the confining mold. When it was attempted to reduce processing time by removing the parts from a hot mold cavity the parts exhibited poor texture, the closed foam cells were perceptible and the part tended to rip and split due to poor hot strength properties.